The present invention relates to folding apparatus, and more particularly to cylinder zig-zag folders such as described in U.S. Pat. No. 3,250,528, issued May 10, 1966 and assigned to the assignee of the present invention. Such folders use a pair of driven cylinders which are mounted with their axes of rotation in parallel adjacent relation. The outer cylindrical walls of the cylinders define a nip which receives the web, and gripper and tucker blades in the cylinder walls alternately fold and draw the web from the nip to deposit it in zig-zag fashion in a stack beneath the cylinders on a moving delivery table.
Folding machines such as that described in the above U.S. Pat. No. 3,250,528 have satisfactorily and accurately folded continuous, cross perforated webs at very high speeds. Further, since the principal motions are rotary, with very little reciprocating action, the theoretical maximum speed of operation would seem to be almost without limit. Limitations are imposed, however, by the paper web itself, since at higher speeds it tends to cling to the surfaces of the cylinders. This problem is solved in part by the use of stripping pins, such as the stripping pins 87 shown in the above-noted U.S. Pat. No. 3,250,528, which help separate the web from the cylinders. However, at higher speeds the tendency of the web to cling to the cylinders can still "bunch" or curl it in the vicinity of the stripping pins.
U.S. Pat. No. 3,980,291, issued Sept. 14, 1976 and assigned to the assignee of the present invention, discloses a cylinder folder construction which improves separation of the web from the cylinders with a series of spaced apart shallow recesses formed around each cylinder surface. The recesses have bottoms slightly depressed with respect to the outer cylindrical walls and are separated by relatively narrow ribs which contact the web and form the sides of the recesses. The area of actual contact between the web and the cylinders is thereby substantially reduced, resulting in a considerably reduced tendency for the web to cling to the cylinder surfaces. The recesses also trap and pressurize air therein, between the cylinders and the web, aiding the subsequent separation thereof.
Nevertheless, while the above inventions have successfully folded continuous webs at increased speeds, a need still remains to operate such folders at even greater speeds. Not only does this call for even better web separation from the cylinders, but also for better and more rapid stack formation. At such speeds the stack tends to "float" on air trapped between the folds, and to be held up by the lack of air above as the stack tries to settle. Sometimes this causes jams or irregular stacks as the folds float too close to the cylinders, or slip over one another on the air cushions between them. Room thus remains for improving still further the separation of the web from the surfaces of the cylinders and for facilitating proper and rapid stack formation as the web is being deposited at high speed in the zig-zag stack on the delivery table.